Silicon carbide is a perennial candidate for use as a semiconductor material. Silicon carbide has a wide bandgap (2.2 electron volts in the beta polytype, 2.8 in the 6H alpha), a high thermal coefficient, a low dielectric constant, and is stable at temperatures far higher than those at which other semiconductor materials such as silicon remain stable. These characteristics give silicon carbide excellent semiconducting properties, and electronic devices made from silicon carbide can be expected to perform at higher temperatures, and at higher radiation densities, than devices made from the presently most commonly used semiconductor materials such as silicon. Silicon carbide also has a high saturated electron drift velocity which raises the potential for devices which will perform at high speeds, at high power levels, and its high thermal conductivity permits high density device integration.
As is known to those familiar with solid state physics and the behavior of semiconductors, in order to be useful as a material from which useful electrical devices can be manufactured, the basic semiconductor material must have certain characteristics. In many applications, a single crystal is required, with very low levels of defects in the crystal lattice, along with very low levels of unwanted impurities. Even in a pure material, a defective lattice structure can prevent the material from being useful for electrical devices, and the impurities in any such crystal are preferably carefully controlled to give certain electrical characteristics. If the impurities cannot be controlled, the material is generally unsatisfactory for use in electrical devices.
Accordingly, the availability of an appropriate crystal sample of silicon carbide is a fundamental requirement for the successful manufacture of devices from silicon carbide which would have the desirable properties described above. Such a sample should be of a single desired crystal polytype (silicon carbide can form in at least 150 types of crystal lattices), must be of a sufficiently regular crystal structure of the desired polytype, and must be either substantially free of impurities, or must contain only those impurities selectively added to give the silicon carbide any desired n or p character.
Accordingly, and because the physical characteristics and potential uses for such silicon carbide have been recognized for some time, a number of researchers have suggested a number of techniques for forming crystalline silicon carbide.
These techniques generally fall into two broad categories, although it will be understood that some techniques are not necessarily so easily classified. The first technique is known as chemical vapor deposition ("CVD") in which reactant gases are introduced into some sort of system within which they form silicon carbide crystals upon an appropriate substrate. Novel and commercially significant improvements in such CVD techniques are discussed in currently co-pending applications which are assigned to the assignee of the present invention, "Growth of Beta-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 113,921, filed Oct. 26, 1988; and "Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 113,573, filed Oct. 26, 1988.
The other main technique for growing silicon carbide crystals is generally referred to as the sublimation technique. As the designation sublimation implies and describes, sublimation techniques generally use some type of solid silicon carbide material other than a desired single crystal of a particular polytype, as a starting material, and then heat the starting material until solid silicon carbide sublimes. The vaporized material is then encouraged to condense, with the condensation intended to produce the desired crystals.
As is known to those familiar with the physical chemistry of solids, liquids and gases, crystal growth is encouraged when the seed or surface upon which a crystal is being formed is at a somewhat lower temperature than the fluid, either gas or liquid, which carries the molecules or atoms to be condensed.
One technique for producing solid silicon carbide when crystal-type impurity is of little consideration is the Acheson furnace process, which is typically used to produce silicon carbide for abrasive purposes. One of the first sublimation techniques of any practical usefulness for producing better crystals, however, was developed in the 1950's by J. A. Lely, one technique of whom is described in U.S. Pat. No. 2,854,364. From a general standpoint, Lely's technique lines the interior of a carbon vessel with a silicon carbide source material. By heating the vessel to temperatures at which silicon carbide sublimes, and then allowing it to condense, recrystallized silicon carbide is encouraged to redeposit itself along the lining of the vessel. Although the Lely process can generally improve upon the quality of the source material, it has to date failed to produce on a consistent or repeatable basis, single crystals of silicon carbide suitable for electrical devices.
Hergenrother, U.S. Pat. No. 3,228,756, discusses another sublimation growth technique which utilizes a seed crystal of silicon carbide upon which other silicon carbide can condense to form the crystal growth. Hergenrother suggests that in order to promote proper growth, the seed crystal must be heated to an appropriate temperature, generally over 2000.degree. centigrade, in such a manner that the time period during which the seed crystal is at temperatures between 1800.degree. C. and 2000.degree. C. is minimized.
Ozarow, U.S. Pat. No. 3,236,780, discusses another unseeded sublimation technique which utilizes a lining of silicon carbide within a carbon vessel, and which attempts to establish a radial temperature gradient between the silicon carbide-lined inner portion of the vessel and the outer portion of the vessel.
Knippenberg, U.S. Pat. Nos. 3,615,930 and 3,962,406, discuss alternative attempts at growing silicon carbide in a desired fashion. The '930 patent discusses a method of growing p-n junctions in silicon carbide as a crystal grows by sublimation. According to the discussion in this patent, silicon carbide is heated in an enclosed space in the presence of an inert gas containing a donor-type dopant atom, following which the dopant material is evacuated from the vessel and the vessel is reheated in the presence of an acceptor dopant. This technique is intended to result in adjacent crystal portions having opposite conductivity types and forming a p-n junction.
In the '406 patent, Knippenberg discusses a three-step process for forming silicon carbide in which a silicon dioxide core is packed entirely within a surrounding mass of either granular silicon carbide or materials which will form silicon carbide when heated. The system is heated to a temperature at which a silicon carbide shell forms around the silicon dioxide core, and then further heated to vaporize the silicon dioxide from within the silicon carbide shell. Finally, the system is heated even further to encourage additional silicon carbide to continue to grow within the silicon carbide shell.
Vodadkof, U.S. Pat. No. 4,147,572, discusses a geometry-oriented sublimation technique in which solid silicon carbide source material and seed crystals are arranged in parallel close proximity relationship to one another.
Addamiano, U.S. Pat. No. 4,556,436, discusses a Lely-type furnace system for forming thin films of beta silicon carbide on alpha silicon carbide which is characterized by a rapid cooling from sublimation temperatures of between 2300.degree. centigrade and 2700.degree. centigrade to another temperature of less than 1800.degree. centigrade. Addamiano notes that large single crystals of cubic (beta) silicon carbide are simply not available and that growth of silicon carbide on other materials such as silicon or diamond is rather difficult.
Hsu, U.S. Pat. No. 4,664,944, discusses a fluidized bed technique for forming silicon carbide crystals which resembles a chemical vapor deposition technique in its use of non-silicon carbide reactants, but which includes silicon carbide particles in the fluidized bed, thus somewhat resembling a sublimation technique.
Some of the more important work in the silicon carbide sublimation techniques, however, is described in materials other than United States patents. For example, German (Federal Republic) Patent No. 3,230,727 to Siemens Corporation discusses a silicon carbide sublimation technique in which the emphasis of the discussion is the minimization of the thermal gradient between silicon carbide seed crystal and silicon carbide source material. This patent suggests limiting the thermal gradient to no more than 20.degree. centigrade per centimeter of distance between source and seed in the reaction vessel. This patent also suggests that the overall vapor pressure in the sublimation system be kept in the range of between 1 and 5 millibar and preferably around 1.5 to 2.5 millibar.
This German technique, however, can be considered to be a refinement of techniques thoroughly studied in the Soviet Union, particularly by Y. M. Tairov; see e.g. General Principles of Growing Large-Size Single Crystals of Various Silicon Carbide Polytypes, J. Crystal Growth, 52 (1981) 46-150, and Progress in Controlling the Growth of Polytypic Crystals, from Crystal Growth and Characterization of Polytype Structures, edited by P. Krishna, Pergammon Press, London, 1983, p. 111. Tairov points out the disadvantages of the Lely method, particularly the high temperatures required for crystal growth (2600.degree.-2700.degree. C.) and the lack of control over the resulting crystal polytype. As discussed with reference to some of the other investigators in patent literature, Tairov suggests use of a seed as a method of improving the Lely process. In particular, Tairov suggests controlling the polytype growth of the silicon carbide crystal by selecting seed crystals of the desired polytype or by growing the recondensed crystals on silicon carbide faces worked at an angle to the 0001 face of the hexagonal lattice. Tairov suggests axial temperature gradients for growth of between approximately 30.degree. and 40.degree. centigrade per centimeter.
In other studies, Tairov investigated the effects of adjusting various parameters on the resulting growth of silicon carbide, while noting that particular conclusions are difficult to draw. Tairov studied the process temperatures and concluded that growth process temperature was of relatively smaller importance than had been considered by investigators such as Knippenberg. Tairov likewise was unable to draw a conclusion as to the effect of growth rate on the formation of particular polytypic crystals, concluding only that an increase in crystal growth rate statistically corresponds to an increase in the percentage of disordered structured crystals. Tairov was similarly unable to draw any conclusions between vapor phase stoichiometry and crystal growth, but pointed out that certain impurities will favor the growth of particular silicon carbide polytype crystals. For example, high nitrogen concentrations favor cubic polytype silicon carbide crystals, aluminum and some other materials favor the growth of hexagonal 4H polytype, and oxygen contributes to the 2H polytype. Tairov concluded that no understanding of the mechanisms leading to these effects had yet been demonstrated.
In Tairov's experiments, he also attempted using silicon carbide single crystals of particular polytypes as the vapor source material and suggested that using such single crystals of particular polytypes as vapor sources could result in particular polytypes of crystal growth. Of course, it will be understood that although the use of single crystals as source materials is theoretically interesting, a more practical goal, particularly from a commercial standpoint, is the production of single crystals from more common sources of silicon carbide other than single crystals.
Finally, Tairov concluded that the treatment of the substrate surface upon which sublimation growth was directed could affect the growth of the resulting crystals. Nevertheless, the wide variety of resulting data led Tairov to conclude that additional unidentified factors were affecting the growth he observed in silicon carbide crystals, and these unknown factors prevented him from reaching a fundamental understanding of the mechanisms of crystal growth.
Therefore, in spite of the long recognized characteristics of silicon carbide, and the recognition that silicon carbide could provide an outstanding, if not revolutionary, semiconductor material and resulting devices, and in spite of the thorough investigations carried out by a number of researchers including those mentioned herein, prior to the present invention there existed no suitable technique for repeatedly and consistently growing large single crystals of desired selected polytypes of silicon carbide.
Accordingly, it is an object of the present invention to provide a method for the controlled, repeatable growth of large single crystals of silicon carbide of desired polytypes.
It is a further object of the present invention to provide a method of growing large single crystals of silicon carbide by controlling the polytype of the source material.
It is another object of this invention to provide a method of growing such silicon carbide single crystals using source materials other than single crystals of silicon carbide.
It is a further object of this invention to provide a method of growing such silicon carbide crystals by selecting source materials having a particular surface area.
It is another object of this invention to provide a method of growing large silicon carbide single crystals by selecting source materials with predetermined particle size distributions.
It is a further object of this invention to provide a method of growing such silicon carbide single crystals using sublimation techniques and in which the thermal gradient between the source materials and the seed is continuously adjusted to maintain the most favorable conditions possible for continued growth of silicon carbide crystals over longer time periods and into larger crystals than have previously ever been accomplished.
The foregoing and other objects, advantages and features of the invention, and the manner in which the same are accomplished will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments and wherein: